I've got quite a few in japanese... I learned most of my japanese just from playing video games. >_>;
Let's see... For Gameboy I have Telefang 1 and 2 Power, Bugsite Beta and Dino Device 1 Red. My sister has all of the opposite versions. XD Besides that I have Pokemon G/S/C and Puzzle Challenge, Medarot 1, 4, Cardrobottle and Navi, and both Bistro Recipe games.
I have a Wonderswan Swancrystal, which was a Japan-only system that competed with the GBC. XD My sister has more games for hers, but so far I've got...
Digimon D-1 Tamers (A proper Digimon RPG)
Final Fantasy IV (The only FF I've really played)
Guilty Gear Petit 2 (A fighting game, pretty much the same as console versions but SD)
Bistro Recipe (Very different from the GBC games)
Riviera: The Promised Land (A really stylish RPG. There's an english remake for GBA and PSP, but I like the Wonderswan version best.)
For Gamecube we got Naruto Gekitou Ninja Taisen (Clash of Ninja) 3 and 4, which just happened to be the only ones not to get english releases once the series got licensed. (Wow, that was a while back...) They're actually pretty great fighting games, even though I'm not into Naruto as much anymore...
I also have Doubutsu no Mori e+, which is a neat Japan-only upgraded version of Animal Crossing. It basically streamlined the game and added a ton of new content and features, some of which made it into the DS game later. XD It's the best one they made IMO.
For DS... I've got Jump Super Stars and Jump Ultimate Stars, which are crossover fighting games featuring all of the big Shonen Jump manga series, and Super Robot Wars W, which is a crossover strategy RPG featuring a ton of different Mecha shows. XD There's no way they're ever coming out in english because of copyrights, which is really a shame because SRW is especially awesome.
We also have some music games and their sequels, namely Daigasso! Band Brothers (a Stepmania sort of game that's
really really good, where you actually play the different parts in the songs) and Osu! Tatakae! Ouendan (the game that was heavily localized into Elite Beat Agents). Besides that, Rockman ZX, and Medarot DS.
I guess that's a lot, but you asked. XD
Here's a picture of most of them.
I think it's safe to say that about half of the games I play right now are in japanese...